Electronic Structure of the Lowest Excited States of Cr(CO)4(2,2‘-bipyridine): A CASSCF/CASPT2 Analysis

Abstract

The visible and UV absorption spectra of Cr(CO)₄(bpy) are interpreted according to CASPT2 calculations based on CASSCF reference wave functions using atomic natural orbital (ANO) basis sets. The excitation energies of the lowest singlet MLCT (metal-to-ligand-charge-transfer) states range between 12 630 and 17 580 cm^-1. They originate in the excitation of chromium d electrons to the lowest π*_bpy orbital of b₂ local symmetry. Dipole transition moments calculated for individual MLCT transitions show that the only transition expected to contribute significantly to the intense absorption band in the visible spectral region is the a¹A₁ → b¹A₁ transition calculated at 17 580 cm^-1. This result agrees very well with the experimental spectra recorded in weakly solvating C₂Cl₄, characterized by a band at 17 700 cm^-1. The low-energy emission band at 12 850 cm^-1 has been attributed to the lowest a³B₂ state calculated at 12 560 cm^-1. The next set of excited states corresponding to 3d → π*_bpy,a2 excitations range between 25 370 and 26 200 cm^-1 for the singlets and between 24 630 and 25 750 cm^-1 for the triplets. The singlet excited states corresponding to d → d excitations are calculated between 29 650 and 37 360 cm^-1. These results show that the intense absorption in the near-UV spectral region originates in strongly overlapping absorption bands due to closely spaced transitions into MLCT (a₂) and dd excited states, respectively. The c¹B₂ excited state corresponding to the 3d_xz → 3d_z² excitation, proposed as photoactive in the mechanism of the efficient CO loss under irradiation at 27 630 cm^-1, is calculated at 36 320 cm^-1 and is far too high to be directly populated in these photochemical studies. Its mixing with one or several low-lying (singlet) MLCT states at the early stage of the reaction path could be responsible for the observed primary reaction. A comparison between the excitation energies of the lowest singlet states of Cr(CO)₄(bpy) and Cr(CO)₄(dab) is reported. The most significant feature is the lowering of the MLCT excited states on going from the bpy-containing molecule to its dab (1,4-diaza-1,3-butadiene) analog.